Stampers are used in injection molding machines to produce substrates for optical disks. One specific type of optical disk stamper is a thermal insulation stamper, which has a laminated structure with a heat-insulating layer to enhance thermal efficiency in injection molding. The heat insulating capability of such a thermal insulation stamper allows high quality and volume production of optical disks.
FIG. 1A is a cross-sectional view illustrating a laminated structure of a typical thermal insulation stamper.
As shown in FIG. 1A, the laminated stamper includes a first metal layer 1, a heat insulating layer 2, a conductive coating 3, and a second metal layer 4. The first metal layer 1 has a finely patterned surface encoding data to be transferred to the optical disk substrate. The first and second metal layers 1 and 4 and the conductive coating layer 3 are formed of nickel and the heat-insulating layer 102 is formed of polymer.
The laminated structure is provided through initial formation of the first metal layer 1 on a master plate, followed by successive depositions of the heat-insulating layer 2, the conductive coating 3, and the second metal layer 4. The conductive coating 3 is formed by sputtering or the like, and acts as a cathode in obtaining the second metal layer 104 through electroformation.
Such a laminated structure is required to have sufficient durability since a stamper is subjected to harsh environmental conditions during injection molding of optical disk substrates.
FIG. 2 illustrates an example of optical disk manufacturing using a typical injection molding machine.
As shown in FIG. 2, the injection molding machine is formed of a stamper 5 and a mold consisting of two sides or plates 6 and 7, one movable and the other fixed. An optical disk substrate is produced through a manufacturing cycle involving the following processes.
Initially, the mold assembly is heated to a control temperature of approximately 100° C. and the mold plates 6 and 7 are clamped together to define an interior mold cavity 8 (process P1).
Then, molten plastic of approximately 300° C. is injected into the mold cavity 8 (process P2).
Subsequently, the mold cavity 8 is cooled to the control temperature of 100° C. so as to solidify the injected plastic (process P3).
After cooling and solidification, the mold plates 6 and 7 are separated to allow the resulting substrate to be removed from the mold cavity 8 (process P4).
In the manufacture of optical disk substrates, such a manufacturing cycle, completing within 10 seconds or less, is repeated a number of times. This means that the stamper installed in the injection mold is subjected to cyclic thermal stress, i.e., rapidly changing temperatures of between 100° C. to 300° C. in each manufacturing cycle. Moreover, the stamper also sustains cyclic mechanical stress due to injection pressure, that is, the injection of material exerting a pressure of 50 Mpa.
With reference to FIG. 1A, as mentioned, the thermal insulation stamper includes the heat insulating layer 2 which is typically formed of polymer. Although the polymer insulator has several advantages, such as high insulation capability with reduced thickness, ease of handling, and reduced manufacturing cost, the thermal insulation stamper having such a polymer insulator is vulnerable to delamination when subjected to thermal cycling in the injection molding.
FIG. 1B illustrates the typical thermal insulation stamper, delaminating under thermal stress after repeated injection molding cycles.
As shown in FIG. 13, the thermally-induced delamination typically occurs at an interface between the polymer layer 2 and the underlying nickel layer 3 in the laminated structure. Taking into account the fact that polymeric material has a coefficient of thermal expansion roughly ten times greater than that of metal, one major factor causing such interfacial delamination is shear stress caused by the difference in thermal expansion between the polymer layer 2 and the nickel layer 3.
It is known that the delamination causes minute deformation of the patterned surface of the stamper. Naturally, the deformation in the mold pattern results in deformation of the molded substrate and the data pattern of the resulting optical disk substrate. According to a recent assessment, optical disks manufactured with a stamper undergoing a number of manufacturing cycles (i.e., over 100,000 shots) suffered degradation of the data pattern at their inner or outer perimeters, while such defects were not observed with a stamper used in production of smaller lot sizes (i.e., 50,000 shots or below).
Thus, it is advantageous to have a durable thermal insulation stamper having a laminated structure with a heat insulating layer that can withstand the harsh environmental conditions encountered in the injection molding of optical disk substrates.